Fix Python 3 bytes-string Error

Since the library currently relies upon use of `str` as a byte type, an error is thrown when running Python 3, which switches the base string class away from `bytes`. The following error is thrown:

```
File ".../apns.py", line 288, in _get_notification
    + payload_length_bin + payload_json)
TypeError: Can't convert 'bytes' object to str implicitly
```

To resolve this, I added a check for the version of Python which switches the zero byte string to an explicitly defined `bytes` object if running Python 3.